As a means for improving the impact resistance of a styrenic resin, there has been proposed a styrenic resin containing a rubber component (styrene-butadiene rubber, polybutadiene rubber) such as high impact resistant polystyrene HIPS. Various techniques have been adopted to improve the impact resistance, in which a rubber dispersed in a HIPS is increased in particle size or the rubber content is raised. According to such methods, while the impact resistance of a shaped article is improved to some extent, the surface gloss thereof or other characteristics are deteriorated. On the other hand, a rubber component of smaller particle size or a lower rubber content improves the surface gloss of a shaped article but deteriorates the impact resistance. Therefore, the external appearance and impact resistance cannot both be held at high levels.
Control of the morphology of the rubber component dispersed in particles in the resin (morphology of particles) is suggested to improve the impact resistance or characteristics related to external appearance such as surface gloss. For example, a rubber particle formed such as to have a small particle size and a microdomain structure of the core/shell type (core/shell structure) is useful for improving the surface gloss of a styrenic resin. However, the formation of a styrenic resin having a core/shell morphology requires a rubber component to be efficiently dispersed in a styrenic resin matrix, consequently limiting the range of rubber components available therefor [e.g., rubber components highly compatible with styrenic resins (styrene-butadiene rubber, etc.)]. Therefore, a styrenic resin having a core/shell structure cannot be produced with a conventional diene rubber. Moreover, when a diene rubber is used, the morphology of the resultant resin is a salami structure with a large particle size, and hence no improvement in the surface gloss of a shaped article thereof.
Moreover, for improvements in both surface gloss and impact resistance, there has been proposed an idea of using a HIPS containing small rubber particles having a core/shell structure in combination with a HIPS containing larger rubber particles having a salami structure. However, this method requires specific and peculiar polymerization conditions and a blending step, and hence the operation much complicated.
On the other hand, there has been known a living radical polymerization process which is a combination method of a living polymerization method whereby a primary structure of a polymer (e.g., molecular weight, molecular-weight distribution) is easily controllable and a radical polymerization method which is less sensitive to impurities or solvents. Though the living radical polymerization process is a radical polymerization, this process enables control of the molecular weight and provides a polymer having a narrow molecular-weight distribution.
Japanese Patent Publication No. 6537/1993 (JP-B-5-6537) corresponding to USP or WO discloses a compound as an initiator for the living radical polymerization of an unsaturated monomer. Japanese Patent Application Laid-Open No. 199916/1994 (JP-A-6-199916) corresponding to USP or WO discloses a polymerization method for a thermoplastic resin having a narrow polydispersity, comprising a step of heating a free radical initiator, a stable free radical agent and a polymerizable monomer compound.
Further, Japanese Patent Application Laid-Open No. 239434/1996 (JP-A-8-239434) corresponding to USP or WO discloses a process for producing a composition containing a vinyl aromatic polymer and a rubber, comprising a step of polymerizing a vinyl aromatic monomer in the presence of a rubber. The literature teaches the presence of a stabilized free radical in the polymerization step. However, considering the amount of the rubber used, the degree of improvement in impact resistance in this process, in other words, rubber efficiency, is low.